Misleading message about extensions

[asarkar]

ghc-options:
- -XViewPatterns

Warning: Instead of 'ghc-options: -XViewPatterns' use 'extensions: ViewPatterns'

However, adding extensions field to package.yaml generates another warning:

WARNING: Ignoring unknown field "extensions" in package description

What works is default-extensions: ViewPatterns.

This ticket is to change the misleading warning message about the extensions field that doesn't exist.